Semiconductor chips typically are connected to external circuitry through contacts on the surface of the chip. The contacts may be disposed in a grid on the front surface of the chip or in elongated rows extending along the edges of the chip's front surface. Each such contact must be connected to an external circuit element such as a circuit trace on a supporting substrate or circuit panel. The rapid evolution of the semiconductor art has created continued demand for incorporation of progressively greater numbers of contacts and leads in a given amount of space. With such closely spaced contacts, the leads connected to the contacts of the chip must be extremely fine structures, typically less than about 0.1 mm wide, disposed at center-to-center spacing of about 0.1 mm or less. Handling and connecting such fine, closely-spaced leads poses a formidable problem.
Leads are commonly bonded to contacts on semiconductor chips or other microelectronic elements by a process such as ultrasonic bonding or preferably thermocompression or thermosonic bonding. In the bonding process, the bonding region of each lead is engaged by a bonding tool which bears on the top surface of the lead in the bonding region and forces the lead downwardly into engagement with the contact. Energy supplied through the bonding tool causes the lead to joint with the contact.
After the lead is bent into a vertically-curved configuration and bonded to the semiconductor chip contacts, the region adjacent the bonding region is formed. This region is commonly referred to as the "heel" of the lead, i.e., the upwardly curving region close to the contacts on the bond side of the lead. The heel of the lead is typically the most fatigue-susceptible region of the lead. Other curved portions of the lead are also susceptible to fatigue.
Methods of making various lead connections involve deformation of the lead, forming curved portions in the leads. Examples include conventional tape automated bonding ("TAB"), and the methods disclosed in U.S. Pat. Nos. 5,489,749, 5,491,302, 5,629,239, and 5,518,964, the disclosures of which are hereby incorporated by reference herein. As further discussed in these patents, leads can be provided on dielectric layers having gaps such that the leads extend into or across the gaps. To form connections, the leads can be bent downwardly towards contacts on another surface. The leads have the bent configuration as depicted in the drawings. The leads may include a polymer layer. Preferably, the polymer layer is absent in the bond region, or any part of the bond region engaged by the bonding tool, to permit sufficient energy coupling between the tool and the bond interface. Combined metal and polymer lead structures are shown in the above-mentioned '749 Patent and in U.S. patent application Ser. No. 08/715,571, the disclosure of which is also hereby incorporated by reference herein.
As described in the aforementioned patents, the leads may extend on either side of the dielectric layer included in the support structure. Thus, the lead may extend on the top surface of the dielectric layer, remote from the chip or other element having contacts to which the leads are bonded. However, the lead may also extend across the dielectric layer on the bottom surface. Also, the support structure need not include a dielectric layer, but instead may include a metallic lead frame which is used to hold leads temporarily and which is removed from the leads during or after bonding.
Connection components formed as discussed above typically have a reduced fatigue life. It is therefore desirable to provide leads with a structure which reinforces the lead, particularly in the most fatigue-susceptible regions of the lead. The most fatigue-susceptible regions of the lead are those regions which were most distorted in the fabrication of the component. Reinforcing at least these regions enhances the fatigue life of the completed assembly. It is therefore desirable to provide a lead structure which resists distortion. It is also desirable to provide a lead structure which is reinforced against fatigue and which promotes more efficient coupling of energy between the bonding tool and the bond interface between the bottom of the lead and the contact. This in turn allows reduced bonding force, bonding energy and/or bonding time, or provides a stronger bond with the same force, energy and time so that connection components can be fabricated more economically.